1. Field of the Invention
This invention relates generally to inkjet printing systems in which an image (characters or graphics, or both) is constructed on a printing medium as an array of many individually computer-controlled inkdrops; and more particularly to a method for printing such an array which minimizes undesired artifacts that arise in the inkjet printing process.
2. Related Art
An inkjet image can be created through any one of myriad different inkjet nozzle firing patterns, implemented by specific program steps of a microprocessor. Such program steps are usually provided in the form of firmware for a general-purpose programmable microprocessor, but fixed arrays of electronic logic elements can be used instead.
Any such firing pattern is called a "print mode". Print-mode variations encompass special uses of a pen or different portions of a pen in coordination with choice of printing-medium advance distance or frequency, special suppressions of firing for specified nozzles, and special choices of nozzle timing (as, for example, during scanning in different directions).
Many different print modes are used to accomplish many different purposes. Such objectives include, for instance, high speed, or more accurate hue for specific printing media, or subtle reduction of the total amount of liquid ink present on a piece of print medium at one time, or (as in the Raskin patent document mentioned above) better alignment of forward and return scans in bidirectional printing, or masking of subtle printing defects--such as arise, for example, from cyclical variations or other imperfections in print-medium advance distance, or from isolated nonfiring nozzles, or from internozzle variations in ink volume.
It is believed that heretofore all print modes known in the art have been devised on the implicit assumption that inkdrops, and (with respect to print quality) assemblages of inkdrops, are symmetrical. Thus it has been taken for granted that a drop deposited on a printing medium by a pen while scanning in one direction (for instance, from left to right) looks the same as a drop deposited by the same pen while scanning in the opposite direction (such as from right to left).
Accordingly it has been implicitly assumed that inkdrop arrays deposited during scanning in different directions look the same--or, to put it in still another way, that image quality at both sides of a drop, or both sides of an assemblage of drops, is the same. If this were true, both sides of every image element would be equally good, or equally bad, so (given mutual alignment of image portions deposited during opposite-direction scans) it wouldn't matter which way the pen moved while printing different parts of each image element.
It has been pointed out, however, that these implicit assumptions are false, or in other words that inkdrops or at least assemblages of them do indeed have good sides and bad sides. Careful inspection and study of inkjet-printed images reveals an image-quality asymmetry, correlated with the direction of pen scanning.
Specifically, each distinct image element is cleaner on the side from which the pen moves toward that element than it is on the side toward which the pen moves from the same element. This difference appears to be due to a tendency of minute amounts of extremely fine spray, associated with firing of the pen, to overshoot the target position very slightly.
Sometimes workers in this field refer to such spray as the "tail" of the inkdrop or as a "secondary dot". In any event it presents the appearance of a slight blurring, irregularity or dirtiness of the associated trailing part of what has been printed.
Evidently this spray, tail or secondary dot results at least in part from lower average speed of fine particles in the direction normal to the printing medium, and therefore longer transit time between the nozzle and the medium. Such lower average speed may result from the processes that generate the spray, or possibly the effectively greater air damping for small particles, or both.
(Even though damping force decreases with area, the effect of such drag is greater for small particles because mass and therefore momentum decrease with volume. Another possible contributor to offsetting of the spray is downwind drift of fine particles in the wake of the pen carriage. It will be understood that the utility of the present invention, to be described shortly, does not rest on the accuracy of any of these speculations.)
After conventional bidirectional printing, the blurring effect is present at both sides of each image element, and so the original asymmetry of print quality for each element is concealed. After conventional unidirectional printing, the blur is present at only one side.
Earlier thinking addressed to mitigating this print-quality defect may have concentrated on cocking or angling each nozzle to somehow align the tail or secondary dot with the main or primary dot. In any event in commercial practice this defect or at least part of it remains.
Still other known print-quality defects have not been satisfactorily addressed in configuring print modes. In particular it is known that, when very wide continuous image areas are printed all at once, creating a large pool of liquid ink, undesired so-called "wicking" and "runners" occur; these are visible small streams of ink, representing paths of ink movement by capillarity through fibers of the printing medium and away from the area where the ink has been deposited.
Heretofore print modes have addressed overinking problems either by (1) subtlely suppressing the total amount of ink on a printed image, or (2) allowing more drying time by dividing each image segment into checker-board-like subgroupings for printing in successive pen scans, the division being arbitrary--that is, without regard for the particular pattern or characteristics of image elements being printed. Both these techniques are ingenious, and they do ameliorate other overinking problems (such as offsetting, and adhesion of adjacent sheets) on which the present discussion does not focus, as well as misalignment between adjacent pen swaths; yet these techniques do have drawbacks.
Suppressing the total amount of ink has some at least slight adverse effects on hue accuracy and color saturation. More importantly--since such ink reduction is willy-nilly, i. e. not correlated, with respect to image characteristics--overall ink suppression may unduly starve some image areas that actually are not subject to wicking.
Dividing image segments for printing in three or more successive scans has the disadvantage of increasing the required printing time. It may do so needlessly in image areas free of wicking and runners.
As noted above, wicking is most serious for broad expanses of continuous inking. On the other hand, when only very narrow portions are printed, edges objectionably reveal the pixel structure conspicuously as a distinct scalloped or serrated pattern, because surface tension is inadequate to pull the ink pool into a more-nearly straight border.
Earlier print modes have not effectively addressed this problem. As will be apparent it tends--whenever present--to be exacerbated by the overall ink-suppression or multiscan image divisions discussed above.
Another known print-quality impairment relevant to the present invention, and to other bidirectional printing systems, is misalignment between scans in opposite directions. This problem is manifested as a narrow line of unprinted or background color (for example white) where adjacent pixel blocks, printed in opposite-direction scans, fail to merge visually into a single apparent image element.
The aforementioned Raskin patent document offers one valuable solution to such problems, though it depends for its full effectiveness upon good consistency of motor speeds in the reverse or retrace direction. That solution can evidently avoid narrow unprinted gaps where blocks of pixels laid down in opposite directions fail to be spaced properly at the regular pixel periodicity.
It may not, however, cure a gap in which failure to visually merge results from inadequate wicking between the two blocks, due to one being already dry. A simpler solution to interscan misalignments, and one which may help to avoid this particular problem, could be useful.
Heretofore these considerations have not been taken into account in devising print modes for best print quality. Thus important aspects of the technology used in the field of the invention are amenable to useful refinement.